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Below is a picture of a pressure gauge mounted on a Freon (R-134a) cylinder for precise recharging of air conditioning systems. It clearly shows a relation between Pressure and Temperature. Is this relation between P and T always true or it applies only to vapor pressure? If the bottle is filled with gas only, no liquid phase present, would the relationship still valid?

Now let's suppose to fill half of my bottle with liquid Freon at a temperature of 70F, seal it and I cool it off to -15F (note -15F is the boiling point at 14.7 PSI). What happens to its volume, vapor pressure, and boiling point?

enter image description here

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  • $\begingroup$ A Google search of vapor pressure will answer many of your questions. $\endgroup$
    – David White
    Commented Oct 20, 2018 at 1:46
  • $\begingroup$ The gauge's purpose is to show the vapor pressure of liquid R-134A freon for any given temperature of the freon. So, yes, in order for the relationship between the temperature and pressure scales on the gauge to be valid, you have to be measuring a container that has at least some liquid R-134A as well as gaseous R-134A (and nothing else). If you seal the container and cool it down to -15F, and -15F is the boiling point at 14.7 psi, then the pressure in the container will be 14.7 psi. $\endgroup$
    – user93237
    Commented Oct 20, 2018 at 2:59

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The vapour pressure of R134a as a function of temperature is readily Googlable. For example there is a table here. Comparison of this data with the picture of the gauge shows that the gauge is showing the relationship between the vapour pressure and the temperature. So it will give the correct readings only as long as some liquid remains in the cylinder.

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